1. Field of the Invention
The present invention relates to a gas seal, and more specifically, to an "O" ring holder for use in a gas seal.
2. Description of the Related Art
In a conventional gas seal, an "O" ring is placed between the stationary seal ring and the stationary seal ring carrier to provide a secondary dynamic seal. The seal ring is typically manufactured from materials having low thermal expansion coefficients, such as carbon graphite, silicon carbide, or tungsten carbide. The ring carrier in such a conventional gas seal is fabricated from metal, such as stainless steel, Inconel 625, or another metal having good corrosion and temperature resistance, and the required strength. These metals from which the carriers are formed have coefficients of expansion approximately three times that of the typical seal ring materials.
The seal rings are mounted within a stationary housing and mounted over a metal balance diameter connected to the stationary housing and centering the seal ring. A clearance exists between the balance diameter and seal ring to compensate for differential thermal growth between the two. For a seal with a 4.625 inch balance diameter, a diametrical clearance of 0.015 to 0.019 inches is common. During operation, the seal assembly becomes hot, with temperatures up to approximately 350.degree. to 400.degree. F. When the assembly heats, the balance diameter expands relative to the seal ring and centers the seal ring.
However, differential thermal expansion between the balance diameter and the seal ring will create variability in the squeeze force on the "O" ring. Typically, a light original radial squeeze is employed of approximately 5% for a 0.139 inch diameter "O" ring cross section. The variability in the squeeze can be as much as .+-.4%. The variation occurs not only from differing expansion rates but also from the stack up of tolerances in the "O" ring and mating components. This variability will adversely effect both "O" ring friction and the dynamic tracking of the stationary seal ring. "O" ring stiffness (i.e., the force preventing opening and closing) of the sealing faces may be as high as 100,000 pounds per inch. This is typical for a five inch diameter "O" ring at 1000 psig.
"O" ring friction is a particularly dominant parameter affecting seal performance during low pressure applications. The friction force imparted by the "O" ring is a much larger percentage of the gas pressure forces acting on the seal faces. Often, premature failure of the seal results from excessive "O" ring friction.
Attempts have been made to eliminate the differential thermal squeeze on the "O" ring by placing the "O" ring between a metallic "O" ring holder and a metallic seal ring carrier. Thermal squeeze is limited by placing the "O" ring between two components of similar material having similar coefficients of thermal expansion.
U.S. Pat. No. 5,039,113 to Gardner, owned by applicants' assignee and incorporated herein by reference, discloses a pusher type non-contacting gas seal incorporating a dynamic "O" ring. As shown in FIG. 7 of this patent, the seal, generally referred to by reference numeral 100, includes a stationary seal ring assembly 102 attached to a machine body 104 and opposing a rotary assembly 106 attached to a rotatable shaft 108.
The seal uses a T-shaped "O" ring holder 110 to minimize drag placed on a stationary seal ring 112 by an "O" ring 114. Dynamic tracking capability is thereby enhanced. The "O" ring 114 is radially squeezed between the "O" ring holder 110 and a seal ring housing 116, both made of metals having similar coefficients of thermal expansion. By having the "O" ring radially squeezed between metals of similar thermal expansion, thermal expansion of the "O" ring holder 110 and the seal ring housing 116 does not create a substantial change in the "O" ring radial squeeze.